Soil nutrients are the basis for plant growth and development, and vegetation plays an important and decisive role in soil nutrient availability and biological processes 16. Plants cultivated on tailings often exhibit stunted growth and withered stems and leaves. This is primarily attributed to the tailings substrate's limited nutrient content, poor water retention capacity, and the absence of vital elements such as organic matter, nitrogen, and phosphorus, which are essential for plant growth 17. Among the various slag mix treatments, the M5 treatment demonstrated superior efficacy in augmenting the height, stem and leaf dry weight, fresh weight, and root dry weight of Pa and Aa. The M2 treatment showed similar effects on Cg, while the M3 treatment notably enhanced the aforementioned metrics in both Sg and Ps. This improvement can be primarily attributed to the conditioner's organic matter content and the abundant nutrients present in the original soil, which together furnish adequate nutrients essential for plant growth. Additionally, water supplementation enhances the substrate's water retention capability, thereby further boosting soil fertility. 18. This correlation was further verified by the positive correlation of plant height, stem and leaf dry weight, stem and leaf fresh weight, and root dry weight with the nutrient elements of the tailing substrate. The main mechanism of action may be that the nitrogen-phosphorus-potassium-source amendments increase the metabolic activity of soil microorganisms, which promotes the soil maturation process of the tailings substrate and facilitates the settlement and growth of plants 19–21. The observations align with findings from earlier research. Specifically, Pardo et al enhanced mine soil using pig manure and compost, noting a marked increase in the contents of water-soluble carbon, water-soluble nitrogen, effective phosphorus, and effective potassium in the treated soil—factors that collectively facilitated plant growth 22,23. However, there was no plant growth in the N treatment at 18 months and 30 months after vegetation restoration. This may be related to the loose structure of the tailings. Solely employing available nitrogen fertilizers, such as urea, exposes the soil to high leaching potential, leading to a nitrogen deficit in subsequent growth stages and resultant seedling mortality 24,25. A larger chemical composition of conditioner and water does not necessarily translate to enhanced plant growth. According to Li et al 26, 5%, 10% and 15% of attapulgite and biochar were added respectively to complex heavy metal contaminated soil, and the combined application of 10% attapulgite and 10% biochar has the best growth-promoting effect on ryegrass.
Soil pH significantly influences soil activity, with changes in microbial substances directly impacting the metabolism, growth, and development of both plants and organisms. The pH level is closely tied to pivotal physical and chemical properties of the soil 27. In this study, the soil matrix pH for Pennisetum alopecuroides (L.) Spreng, Campsis grandiflora (Thunb.) Schum, Setaria glauca (L.) Beauv, Periploca sepium Bunge and mugwort consistently exceeded 7.5, indicating alkalinity. Furthermore, the nutrient levels of these soils fall within an optimal pH range. The increased alkalinity of tailings primarily arises from the anions of alkaline substances absorbing the tailings' cations 28. The inherent low pH of the original soil can either neutralize the tailings' alkalinity or, possibly, the selective enrichment of alkaline ions within the plant rhizosphere can reduce soil alkalinity 29. From another perspective, the application of compound amendments improves the pH-improving effect of herbs and is beneficial to tailings remediation.
Soil nutrients are crucial for plant growth and development 30. Plants can either directly absorb mineral nutrients like nitrogen, phosphorus, and potassium from the soil or assimilate them after transformation 31. The composite improver enhanced the levels of macronutrients in the tailings, including TK, AK, TN, and AP. This enrichment is likely attributed to the combined effects of the original soil, nitrogen fertilizers, and water, which augmented the nutrient content, specifically N, P, and K 32. Relative to the CK group, the AK content in planting areas treated with compound amendments exhibited an upward trajectory. The primary source of this potassium is the potassium-bearing minerals in the soil's parent material 33. However, as the soil's pH decreases, its potassium fixation capability diminishes. Consequently, there is an elevation in the tailings' AK content, and the slow-releasing potassium in the tailings is transformed into readily available forms. Simultaneously, the soil experiences an increase in H+ concentration, non-specifically adsorbed potassium in colloids, water-soluble potassium, and exchangeable potassium, all influencing plant responses 33. In addition, the contents of total phosphorus and available phosphorus in each plant planting area were lower than those in the CK treatment. This decrease may be attributed to the shift in soil pH affecting phosphorus content. As water-soluble phosphorus is progressively released and assimilated by plants, the effective phosphorus content of the tailings diminishes 34. Moreover, in alkaline soils, phosphorus readily reacts with calcium, predominantly forming low-solubility calcium phosphate salts, which curtail the availability of phosphorus and result in a reduced effective phosphorus content 35.